Buggy code:
public class QuickSignatureCommand extends AbstractHandler { private IFile file = null ; private String resource ; private String xpath = "" ; private String signatureType ; private Keystore keystore ; private String keyFile ; private char [ ] keystorePassword ; private String keyName ; private char [ ] keyPassword ; private String canonicalizationAlgorithm ; private String transformationAlgorithm ; private String messageDigestAlgorithm ; private String signatureAlgorithm ; private String signatureId ; private boolean completePrefs = false ; private ExecutionEvent event ; private ITextSelection textSelection = null ; public Object execute ( ExecutionEvent event ) throws ExecutionException { } private void createSignature ( ) { } private void signData ( final IDocument document ) { } private boolean loadKeystore ( ) throws Exception { } private void getPreferenceValues ( ) { } private boolean checkPreferences ( ) { final String title = Messages . QuickSignatureCommand_4 ; <START_BUG> final String prefId = "org.eclipse.wst.xml.security.ui.preferences.Signatures" ; <END_BUG> int result = 2 ; if ( ( ( resource ) == null ) || ( "" . equals ( resource ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , QuickSignatureCommand_11 ) , prefId ) ; } else if ( ( ( ( resource ) != null ) && ( "xpath" . equals ( resource ) ) ) && ( ( ( xpath ) == null ) || ( "" . equals ( xpath ) ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , QuickSignatureCommand_12 ) , prefId ) ; } else if ( ( ( signatureType ) == null ) || ( "" . equals ( signatureType ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , QuickSignatureCommand_13 ) , prefId ) ; } else if ( ( ( keyFile ) == null ) || ( "" . equals ( keyFile ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , QuickSignatureCommand_14 ) , prefId ) ; } else if ( ( ( keyName ) == null ) || ( "" . equals ( keyName ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , QuickSignatureCommand_15 ) , prefId ) ; } else if ( ( ( canonicalizationAlgorithm ) == null ) || ( "" . equals ( canonicalizationAlgorithm ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , QuickSignatureCommand_16 ) , prefId ) ; } else if ( ( ( transformationAlgorithm ) != null ) && ( "" . equals ( transformationAlgorithm ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , QuickSignatureCommand_17 ) , prefId ) ; } else if ( ( ( messageDigestAlgorithm ) == null ) || ( "" . equals ( messageDigestAlgorithm ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , QuickSignatureCommand_18 ) , prefId ) ; } else if ( ( ( signatureAlgorithm ) == null ) || ( "" . equals ( signatureAlgorithm ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , QuickSignatureCommand_19 ) , prefId ) ; } else if ( ( ( signatureId ) == null ) || ( "" . equals ( signatureId ) ) ) { result = showMissingParameterDialog ( title , NLS . bind ( MissingParameter , MissingSignatureId ) , prefId ) ; } else { completePrefs = true ; } if ( result == 0 ) { completePrefs = false ; getPreferenceValues ( ) ; checkPreferences ( ) ; } return completePrefs ; } private boolean checkPasswords ( ) { } private int showMissingParameterDialog ( final String title , final String message , final String prefId ) { } }

Fix:
final String prefId = "org.eclipse.wst.xml.security.ui.preferences.Signature" ;